Bug 1195142 patch 1 - Set CSS_PROPERTY_CREATES_STACKING_CONTEXT for the opacity property. r=BenWa

This isn't actually needed for the only caller (which ensures that
frames with will-change: opacity create a stacking context), since
nsIFrame::BuildDisplayListForChild checks HasOpacity, which checks for
NS_STYLE_WILL_CHANGE_OPACITY.  However, it's good to have the bit set
for consistency in case we use it elsewhere.

--HG--
extra : commitid : 2mKHVXRkjZL

layout/style/nsCSSPropList.h

@@ -2663,7 +2663,8 @@ CSS_PROP_DISPLAY(
     Opacity,
     CSS_PROPERTY_PARSE_VALUE |
         CSS_PROPERTY_APPLIES_TO_PLACEHOLDER |
-        CSS_PROPERTY_CAN_ANIMATE_ON_COMPOSITOR,
+        CSS_PROPERTY_CAN_ANIMATE_ON_COMPOSITOR |
+        CSS_PROPERTY_CREATES_STACKING_CONTEXT,
     "",
     VARIANT_HN,
     nullptr,